Investigating viral evolution: Insights into the origins of obligate intracellular parasites

Viruses are obligate intracellular parasites composed of genetic material (either DNA or RNA) enclosed within a protein capsid and, in some cases, an additional outer lipid envelope. Exhibiting both living and non-living characteristics, viruses remain a microbiological enigma, with scientists continually debating their classification and origins.

Some scientists argue that viruses are living due to their ability to evolve and adapt, whereas others classify them as non-living because viruses lack cellular structure and the ability to reproduce independently of the host cell. While the question of whether viruses are living or non-living is an intriguing philosophical debate, it has little bearing on how we study and understand viruses. In contrast, by investigating the ancient origins of viruses, we can gain insight into how viruses have shaped the evolution of life on Earth, deepening our understanding of the natural world and informing modern research in virology and medicine.

There are three main theories for the origin of viruses: the progressive or escape hypothesis, the regressive hypothesis, and the virus-first hypothesis.

According to the progressive hypothesis, viruses originated from mobile genetic elements within host cell genomes that evolved the ability to move between cells. Retrotransposons, virus-like genetic material capable of moving and integrating into different locations within a genome, constitute 8% of the total human genome. This suggests that acquiring extra structural proteins may have allowed these genetic elements to move between cells as infectious agents.

The regressive hypothesis proposes that viruses originated from free–living cellular organisms, like bacteria or other microbes. After gradually losing essential genes as they adapted to a parasitic lifestyle, they eventually become completely dependent upon their hosts for replication and survival. This hypothesis is best supported by the discovery of larger viruses, such as nucleocytoplasmic large DNA viruses (NCLDVs), that display greater complexity than other viruses.

The third theory for the origin of viruses is known as the virus-first hypothesis, which suggests that viruses predate cellular life. Proponents of this theory argue that viruses existed in a precellular world as self-replicating genetic elements. This theory is supported by the current existence of viroids, simple self–replicating molecules of RNA with only 250 to 400 nucleotides. These viroids highlight the plausibility of an ancient, virus-like stage in the origin of life.

The evolutionary origin of viruses remains unknown, with continual debate amongst experts in virology. For example, Claudiu Bandea, at the Center for Disease Control and Prevention, argues viruses may be the result of “parasitic species that fused with their host cells, by a process in which the parasitic species fused with the host’s cellular membrane,” thereby losing its cellular structure and resembling what we now recognize as a virus. While Bandea’s “fusion model” for the origin of viruses remains unconfirmed, further research on viral genes could shed light on whether viruses evolved from cellular ancestors or emerged as distinct genetic entities. 

By unraveling how viruses emerged and evolved, we will gain a more complete understanding of virus-host relationships, their influence on cellular evolution, and their ecological significance. Advances in genomic sequencing and molecular analysis may provide crucial insights into these questions, helping to reveal the true evolutionary history of viruses.